A cermet in which the particles of a hard compound phase such as a carbide and a nitride are bound by a metal phase has been used as a material for modifying the surface properties of substrates. A spraying method has been used conventionally for the formation of a cermet coating. In a spraying method (for example, a high velocity flame spraying process), the raw material powder is melted, and the melted particles are sprayed onto a substrate to be deposited on the substrate. However, for example, in the case of a WC—Co coating that uses tungsten carbide and cobalt as materials, the tungsten carbide-cobalt (WC—Co) coating prepared by a thermal spraying process forms, within the coating, not only a hard WC phase and a Co binder phase that have a composition of the raw powder, but also η phases such as Co3W3C and Co6W6C, and a W2C phase, by the dissolution of the WC phase into the Co phase or the reaction of the WC phase with oxygen, at the time of the process. Further, since the melted particles are rapidly cooled on the substrate, the structure of the binder phase is a structure composed of amorphous materials. As a result, although cermet coatings such as the WC—Co coating formed by a conventional thermal spraying process have excellent hardness, the fracture toughness was low, and the occurrence of damages such as partial detachment and chipping has been a problem when an instantaneous load is applied locally (for example, Non-Patent Documents 1 and 2). Further, in the WC—Co coating containing such degradation phases at the time of the process (the η phase and W2C phase described above), since it contains a W—C—Co based material that is excluded from the region of the two-component system with WC and Co as the end components, it is impossible to return to a two-phase state of WC and Co even when the coating is subjected to a heat treatment.
On the other hand, by a new process called the warm spray process, it has become possible to allow the WC—Co raw powder, without melting, to collide with the substrate at a high speed while being in a solid phase, thereby suppressing the thermal degradation during the process (for example, Patent Documents 1 and 2, and Non-Patent Document 3). The coating obtained by this process is constituted by a WC phase and a Co phase that maintain the structure of the raw material powder, while the formation of a degradation phase is suppressed. However, although the fracture toughness of the coating obtained by the warm spray process is superior to that of the coatings obtained by a conventional high velocity flame spraying process, it has been far inferior, as compared with that of the sintered bulk body of WC—Co which is produced by a hot press or the like (Non-Patent Documents 3 and 4). The reason for this is that because the particles fly at a very high speed and collide with the substrate during the process, an extremely large plastic deformation occurs within a Co phase serving as a binder phase, and the crystal structure is disturbed and mostly becomes amorphous.